Why not let the TD seat players? The TD can randomly seat players automatically as they buy in to the tournament.

Or, if you must do it yourself, you can register players from the Tables page. Either use the hotkey Ctrl+A to add players to the tournament (you may need to enable hotkeys on that tab, see below), or use the Dashboard. Enable the Dashboard by right-clicking in the top section of the Settings window and selecting "Show Dashboard". You can add players by pressing the "+" button in the Buy-ins section of the dashboard.

To enable hotkeys on the Tables tab, select the Preferences tab, then select the Behavior category. Then go down about 25 or so lines and make sure to check "Allow hotkeys on all Settings tabs that do not have inputs".